1. Field of the Invention
The present invention relates to a method for forming an orientation film wherein a mixture of a liquid crystal compound and an organic compound which is oriented to a certain direction by utilizing orientation of the liquid crystal compound to the certain direction caused by magnetic field or voltage application is coupled to a silane coupling agent chemically adsorbed on a substrate surface.
2. Description of the Related Art
Generally, a liquid crystal display device is manufactured in a manner comprising the following steps:
Transparent stripe-shaped electrodes made of a material such as indium tin oxide (I TO) are formed on transparent substrates made of a material such as glass, whereafter an orientation film is formed thereon. Then such two transparent substrates are disposed opposite to each other so that cross portions of the transparent electrodes are arranged in a matrix form. A spacer is disposed between the peripheral portions of both the substrates in order to provide a gap therebetween and the peripheral portions of both the substrates and the spacer are bonded with an adhesive agent. A liquid crystal is injected through an inlet into the gap and the inlet is sealed, whereby a liquid crystal device is formed. Thereafter, a polarization plate is disposed on both sides of the liquid crystal device so that polarization axes through both the sides thereof make a right angle with each other. Finally a liquid crystal display device which functions as a display device is completed by backlighting from one of the sides thereof and applying a voltage thereto. Namely, displaying is performed by utilizing a character that when a voltage is applied to the liquid crystal device, an orientation phase of liquid crystal molecules varies, which results in variation of a transmitted light quantity.
It is indispensable to uniformly orient injected liquid crystal molecules to a certain direction in order to obtain a liquid crystal device with a high contrast. The uniform orientation depends on an used orientation film. There are found several methods of forming an orientation film. For instance, in rubbing method, a high molecular film of polyimide, polyvinyl alcohol or the like is applied on a substrate having electrodes thereon by use of a spinner. Thereafter the surface of the high molecular film is rotationally rubbed toward a certain direction with a roller wrapped with a cloth made of nylon. As a result of this process, distortion anisotropy is provided, which makes it possible to form an orientation film wherein the longitudinal axes directions of liquid crystal molecules direct toward the rubbing direction.
Further, a method for forming a monomolecule adsorption film made of a silane coupling agent disclosed in Japanese Unexamined Patent Publications JP-A 66-7913 and JP-A 66-186818 (KoKai) is well known. In this method are utilized several kinds of silane coupling agents which have a normal carbon chain, and a monochloro or trichloro silane group at one end thereof which is to be bonded on a substrate having electrodes, or have a compound at one end thereof whose structure is similar to that of a liquid crystal molecule to be injected into a liquid crystal device. These silane coupling agents are applied to the substrate with electrodes in order to be adsorbed on the entire surface of the substrate by a chemical adsorption method, whereby a monomolecule adsorption film made of the silane coupling agent is formed.
Further, according to other method (disclosed in January J. Appl. Phys. Vol. 29 No. 9, L1689 (1990)) for forming a monomolecule adsorption film, a mixture of silane coupling agents with different lengths of normal carbon chains is employed to control a pretilt angle of a liquid crystal molecule.
However, among the above-mentioned methods, the rubbing method is an easy method of rubbing the surface of the polymer film with a cloth wrapping around a roller. The method is widely employed in industries due to the excellent mass productivity and high reliability thereof, to be sure, but it has several disadvantages as follows.
Electrostatic break down might occur in a thin film transistor for driving due to static electricity caused by rubbing with the cloth in a case of an active matrix type of liquid crystal device in which a metal-oxide-semiconductor field-effect transistor (MOS-FED) is integrated. Further, the ununiform thickness of the substrate, abrasion of the rubbing cloth or rubbing cloth dusts deposited on the polymer film causes ununiformity of the rubbing intensity, which might also result in ununiform orientation. A method without rubbing has been investigated in order to eliminate these disadvantages and a chemical adsorption method of a silane coupling agent has been proposed. In particular, satisfactory orientation can be achieved in a monomolecular adsorption film made of a silane coupling agent containing a compound whose molecular structure is similar to that of a liquid crystal molecule, because a liquid crystal injected into a liquid crystal device is penetrated into a gap between molecules of the compound with the structure similar to that of the liquid crystal. This method almost solves the problems in the rubbing method. However, while vertical orientation of the liquid crystal can be achieved due to the action of the silane coupling agent as an vertical-orientation agent, such orientation of liquid crystal molecules as inclined orientation to a certain direction and horizontal orientation cannot be achieved in this method unlike the rubbing method.
When a mixture of silane coupling agents having a normal carbon chain whose length is different from that of each another is employed for a monomolecular adsorption film, a pretilt angle can be controlled by varying a mixing ratio of these silane coupling agents. The orientation direction of the mixture of the silane coupling agents, however, is not controlled also in this case and therefore it is necessary to control the orientation direction of the mixture of the silane coupling agents in a manner such as rubbing, on or after the formation of a monomolecular adsorption film, in order to orient a liquid crystal to a certain direction.